Solar photovoltaic module safety shutdown system

ABSTRACT

A PV system may be used in case of emergencies. Each individual photovoltaic module receives a signal to determine if it is allowed to be operational or must shut down. Modules by default are shut off and safe to handle, absent the signal and in the presence of light.

CROSS-REFERENCE TO RELATED APPLICATION

This application is related to U.S. Provisional Application Ser. No.61/252,985, filed on Oct. 19, 2009, hereby incorporated by referenceherein.

BACKGROUND

FIG. 1 describes a typical photovoltaic (PV) grid-tied 100 or off-grid110 system. A PV system consists of a number of modules 101; each moduleby itself generates power when exposed to light. A series of modules iswired together to create a higher voltage string 102. Multiple PVstrings may be wired in parallel to form a PV array 103. The PV arrayconnects to a DC-disconnect switch 104, and the DC disconnect switchfeeds power to a grid-tied inverter 105 which converts the DC power fromthe array to AC power for the grid.

Off-grid systems 110 connect the PV array 103 to the DC disconnect, andon to a battery charger 111, which stores the electrical energy inbatteries 112. Off-grid residential systems typically use an off-gridinverter 113 that produces AC electricity for AC loads connected to anAC main panelboard 106.

Inside a silicon cell based module 200, shown in FIG. 2, there is aseries of photovoltaic cells 201, the basic building block in solarelectric systems. Each cell is producing approximately 0.5 volts and afew amps (e.g. 5 A). The PV cells are also wired in series and inparallel within the module to achieve a desired voltage and current, andeach module has a positive and negative module terminal 202 to connectto the PV system. A typical module used in a residential or commercialpower generating system will produce in the order of 18-50V DC at 90-200W at its electrical connectors. There are two terminals one positive andthe other negative. Arrays used in residential installations willtypically produce power in the range of 2 KW-10 KW with voltages up to600V DC (grid-tied). The module voltage and power output is true forother module architectures such as thin-film (CdTe, CIGS, etc.).

When a PV array is installed and operational, the PV system generatespower whenever there is light present. Furthermore, it is impractical todisable the system beyond shutting off the AC mains or the DCdisconnect. Once wired, the array itself is never able to fully shutdown in the presence of light even with the DC disconnect in the openposition. The string wiring connecting all the modules in series, thewiring to the DC disconnect, and the array will all continue to generatelethal levels of voltage when exposed to light.

In the case of a damaged array from fire or natural disaster, an open(non-insulated) wire of the array's circuits may present itself. Theexposed circuits provide a higher likelihood of an unintended electricalcircuit path to ground (ground fault), and a human can become a part ofthis path to ground either by touching or through exposure to water.With a human body in a ground fault circuit it is very likely to belethal. The National Fire Protection Association (NFPA) 70E defines “lowvoltage” somewhere near ˜50V. This low voltage is the threshold whereone is able to generally survive a shock and “let go” (˜9 mA). PVsystems are well above this level. This poses a serious and very realproblem for firefighters when they encounter a building on fire with aPV array.

Even an operational and properly insulated system poses a potentialproblem for service technicians in the case of a PV array in need ofservice. In the case of the need to replace a defective module theperson may be exposed to high voltages even with the DC disconnect inthe “off” or “open” position.

In the case of earthquakes, floods, or other natural disasters,partially destroyed PV systems pose a threat to the occupants of astructure and any rescue personnel, especially untrained civilians.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 illustrates two embodiments of a solar photovoltaic (PV) systemwith all the major system components;

FIG. 2 illustrates the elements of a current PV module;

FIG. 3 illustrates an embodiment of a PV system with the additionalsystem-level components;

FIG. 4 illustrates the elements within the system-monitor function;

FIG. 5 illustrates the two possible methods to control an individualmodule;

FIG. 6 illustrates a module embodiment utilizing an electromechanicalrelay as the means to control the module;

FIG. 7 illustrates a module embodiment utilizing a transformer andtransistor as the means to control the module;

FIG. 8 illustrates a module embodiment utilizing an opto-isolator andtransistor as the means to control the module;

FIGS. 9 and 10 illustrate a module embodiment utilizing an FET driverand transistor as the means to control the module; and

FIG. 11 is a depiction of the back side of one embodiment of a solarmodule.

DETAILED DESCRIPTION

Typically a firefighter disables power to a dwelling at the main ACpanelboard of a home prior to dousing it with water. Shutting off powerto the AC main panelboard automatically disables every individual modulein a photovoltaic (PV) array in one embodiment. In natural disasters theAC mains of a building will likely be off, again making the PV systemsafe in one embodiment. Additional systems allow for automatic shutdownin the case of natural disasters.

A System-Monitor device 301 creates a “System-On” signal 302, which eachindividual solar module receives to activate itself and operatenormally, as shown in FIG. 3. This signal controls (i.e. enables ordisables) each module individually. Disabling a module can be achieved,for example, by shorting out the module or by opening up the stringcircuit that connects all the modules together. The System-Monitordevice is connected to the AC power of the system, and when the AC poweris off, the “System-On” signal is no longer “true.”

Additionally, a manually operated switch 303 that is key operated mayalso be used to disable the PV system and each individual moduletherein, in some embodiments. The manual switch may be used byfirefighters as well as service technicians to disable all the modulesindividually. It includes visual feedback 305 to indicate its state.

The System-Monitor 400, shown in FIG. 4, generates the System-On signalfor the modules to begin operation. In one embodiment this particularcomponent is a simple step down transformer 401. Typically, 240 V ACfrom the AC main panelboard is connected to the primary coil. Thetransformer generates a low voltage (e.g. ˜12V AC 60 Hz) signal pair 402on the secondary coil. An active (e.g. 12V AC) signal indicates to themodules that the AC grid is on, or “System-On” is true. The activesignal may be fed via a set of wires that is then routed to each module,for example, by “daisy chaining” the System-On signal to every module.The System-Monitor device may have a manual switch 403 with akey-lockout that disconnects the step down transformer from the AC poweroperated by anyone who needs to disable the PV system.

The System Monitor may employ internal protection fuses 404 for faultconditions. In the case of an off-grid system, the off-grid inverter 405supplies AC power in order for the System-Monitor to operate. Dependingon code or safety requirements, one leg of the System-On signal may bebonded to ground with a conductor 410.

For natural or man-made disasters, a motion, water or heat sensor andswitch 411 may automatically disable the “System-On,” for example, incase of earthquakes, floods, or fires. Those skilled in the art ofelectronic or electrical design recognize the many options to implementsuch a sensor switch.

FIG. 5 illustrates one embodiment of a mechanism to control the module'spower production. Solar photovoltaic module junction box 506 may includea logic element 501 and module switch 502 may be part of the moduleassembly 500 (e.g. inside the module junction box) or be a separateelement wired to the module (not shown). The System-On signal may beconnected to the logic element with a twin lead connector 503. Themodule terminals 504 deliver direct current (DC) potential to othermodules to form a string. The module switch 502 can be in series withthe PV cells 505 and one of the module terminals, in this case theswitch 502, disconnects the PV cells from the array. In a secondembodiment, the module switch 510 can be in parallel to the PV cells,connecting to both module terminals. The junction box 506 may be aphysical box that is secured to or integrated with a photovoltaicmodule. It may be attached by the module manufacturer at the time ofmanufacture or thereafter by third parties, in some embodiments.

Each of the module switches is electrically isolated from other moduleswitches since each operates at a different voltage potential. This isdue to the series wiring of the modules into a string and the fact thatall the module switches share a common signal “System-On”. A givenmodule switch in a given array may be operating at a high potential(e.g. 400V) to ground, and the next module in the string at 350V, and soon, assuming each module generated 50 Volts (DC). Electrical isolationbetween the common System-On signal and the module switch can beachieved a number of ways including but not limited to AC transformercoupling, or optical coupling inside the logic element 501.

The logic element and switch circuitry can be designed in a number ofways. Those skilled in the art of electronic circuit design willunderstand the proper selection of the individual components, the detailof which is left out for clarity.

The electro-magnetic relay-based system uses electromechanical systemsfor isolation and switching. The signal System-On has enough power toenergize a standard AC relay coil. The signal is operating at a voltageconsidered safe to humans (low voltage, e.g. 12V AC). To control amodule using a relay, the switch may be in series or in parallel withthe PV cells.

FIG. 6 shows the circuit of a module assembly 600 with the module switchin parallel. In a system where the signal System-On is true (e.g. 12VAC) the energized coil 602 moves the normally closed (NC) contact 601 ofthe relay 603 to open up and allows the cells to produce power at themodule terminals. The power from the System-On signal connects to therelay's coil through the two-contact System-On connector 605, and theisolation between the System-On signal and the module switch is providedinherently between the relay coil and the relay's contact.

FIG. 7 shows the circuit of a module assembly 700 utilizing a smalltransformer 701 along with a few other components and a transistor 702to perform the logic element and module switch functions. A basictransformer AC couples the System-On signal, present at the connector703, through a primary coil to a secondary coil. The primary andsecondary coils provide the needed isolation. The coupled and isolatedSystem-On signal is now converted to a DC control signal, for example,through a 4-diode rectifier 704, and the rectified AC ripples arereduced with a capacitor 705. This circuit provides a positive voltageof sufficient level to turn on a power MOSFET transistor 702. The MOSFETsource terminal is connected to the negative terminal of the first in aseries of cells 706, and the MOSFET drain terminal is connected to themodule negative terminal wire 707.

When the System-On signal is false (0V AC), the transistor is off due tothe gate voltage (Vgs) being zero, and the module is disconnected fromthe other modules in the array. With the signal present the transistorwill be on and it will close the circuit with the other modules in thestring. To ensure the transistor turns off without a system-On signal, aresistor 708 discharges the capacitor.

The transformer in the previous example can be replaced with anopto-isolator component, as shown in FIG. 8. The System-On signal at theSystem-On connector 801 is converted to a DC voltage with a dioderectifier 802 and capacitor 803. The DC voltage is current limitedthrough a series resistor 804 to operate the opto-isolator's 805transmitter (LED). The light energy will activate a photosensitive photodetector (e.g. transistor) in the opto-isolator, the light is providingthe electrical isolation. When light is present the opto-isolator'stransistor is conducting current otherwise not. The opto-isolatortransistor controls a MOSFET transistor 806, able to handle the modulepower loads. The N-channel MOSFET is by default off (or open) since thegate is pulled down to the same level as the source with a resistor 807.When the opto-isolator's transistor is on it will raise the voltage ofthe MOSFET's gate close to the level present at the positive terminal ofthe last cell in the module 808 if light is present. The MOSFET will bein fully saturated mode and “on”, connecting the negative terminal ofthe first cell 809 to the module's negative terminal 810 allowing themodule current to flow through the array. The voltage feeding the gatemay need to be limited to protect the MOSFET depending on the choice ofcomponents; this can be achieved with an additional resistor 811.

In FIG. 9, solar module 900 junction box 911 utilizes a photovoltaicMOSFET (PV PET) driver 901 as the isolation function. The System-Onsignal is converted to DC through the rectifier 902, capacitor 903, andis current limited through a resistor 904 as it drives the lighttransmitter (LED) of the PV FET driver. The light energy will beconverted to by the PV FET driver's photodiodes to a DC voltage ofsufficient voltage to directly drive the MOSFET transistor 905 to afully saturated mode. This in turn will connect the negative terminal ofthe first cell 906 to the negative module terminal 907. A resistor 909will guarantee that the transistor will be off by default, bydischarging any energy stored from leakage or stray capacitance. Thepositive module terminal 912 is coupled to cells 908. The System-Onconnector 910 may use twin leads.

FIG. 10 illustrates another design, which has very few components forreliability and low cost. The System-On signal current (via connector1007) is limited through resistor 1002 and drives the photo-diode of theFET photovoltaic driver 1001 in the solar module 1000 junction box 1008to generate light energy for half of the AC cycle. The light energy isconverted by the FET-driver to a DC voltage, which is applied to theMOSFET transistor 1004. The inherent gate capacitance of the MOSFET issufficient to store the needed voltage to turn on the FET for the entireAC cycle, thus eliminating any gate charge storage device. A resistor1003 turns off the FET to bring it to desired default state of “off” bydraining the FET gate charge when the System-On signal is not present.When the System-On signal is present, the transistor is on or fullysaturated. This in turn connects the negative terminal of the first cell1005 to the negative module terminal 1006.

Finally, referring to FIG. 11, the back side of a solar module, such asthe solar module 1100, is depicted. The back side is the side which isnot exposed to receive solar energy. The back side of the module 1100may include a back sheet 1101. In one embodiment, the junction box 1105may be integrally formed within the back sheet 1101. In otherembodiments, it may be a physical box, such as a plastic electrical box,accessible through the back sheet 1101. A pair of leads 1106 may beprovided with a positive terminal 1103 and a negative terminal 1104 toconnect a DC potential to the neighboring modules into a string. Asecond pair of leads 1107 may be provided to “daisy chain” the System-Onsignal from one module to the next. The connectors 1108 for theSystem-On signal contain two contacts each.

Another approach is to deliver the signal representing “System-On” as alight signal to the modules. This example requires a modification to theSystem-Monitor device, which will be sending light instead of an ACsignal. Each module receives a fiber optic cable and the light receivedis converted to a voltage as in the case of the MOSFET driver through aseries of photo diodes to a voltage level sufficient to turn on the FET.

Those skilled in the art of electronics can appreciate the possiblevariations of connecting a common signal (System-On) with some form ofenergy such as an AC or DC voltage, radio waves, or light to an isolatedlogic element. The logic element in turn drives a module switch thatenables power output from the module. The Switch itself may also beintegrated into one of the cells in the series (gated cell). Furthermorethe circuit that controls the PV module may be part of the module or aseparate system component that the module will plug into. The previousexamples illustrate a few of the possible ways to implement theprinciple idea.

In systems with a number of distributed inverters, one for each module,there is no equivalent of the DC disconnect switch, and by turning offthe AC mains the PV system will shut down if the micro-inverters areoperating as expected. However the ability to shut the PV system down bydisabling the power generated from the module itself via the SystemMonitor device (using the manual lockout switch) provides an additionalsafety measure and more importantly a consistent and clear visual meansto firefighters to ensure that the PV array is indeed off. It alsoprovides a safe and lockable means to people servicing the modules.

Currently a module is “live” the moment it leaves the module factory;there is no “off” switch. Like a charged car battery, PV modules aredangerous to the untrained, and able to generate power. Once an array iswired into a PV system it is a permanent installation and is not evertypically disconnected. The array wires pose a particularly lethal levelof power to people since the voltages are typically 200-600V, which byNFPA NEC (National Electric Code) definition is well above “low voltagesystems”. The only practical means of switching off an array is at thesingular DC disconnect point where the lethal voltage levels are presenteven when switched off. Firefighters are trained to shut off the DCdisconnect and the AC mains to a building, however even after both ofthese actions occur the power generated by the array continues to bepresent in the array, within the modules, and the wiring on the roof orinside the home leading up to the DC disconnect. If a firefighter wereto use an axe to ventilate a roof, cut a wire, cut into a module, ordouse a broken array with water—the firefighter would be exposed to highvoltages. A path of lethal current to (earth) ground will exist.Additionally if a PV service technician were diagnosing a faulty arrayfor ground faults, or replacing a broken module, this person will beexposed to very high voltages, requiring very careful conduct withoutany mistakes to remain safe. Electricians prefer in all cases to “lockout and tag out” any circuit they are working on, however a PV arraycannot be shut down by any practical means.

By installing a switch in each module it is possible to deactivate eachindividual module to a level where the voltages will be in the order of18-50 volts or less. At these levels it is safe to handle the modules orany components of the array.

Each module receives a “System-On” signal, a corresponding logicelement, and a switch in or near the module will perform the control ofeach individual module. These may take the form of simple coils andrelays or in other embodiments optical and electronic components. Thereliability or cost of these simple components does not pose a costburden nor a reliability challenge for module manufacturing. Each modulehas a junction box with a few electronic components in it today(diodes), and this shutdown system can add a few more to the module. Inaddition to the module switch the PV system may use a System-Monitordevice. This function may be built into the grid-tie inverter or theoff-grid battery charger to lower overall parts and costs.

References throughout this specification to “one embodiment” or “anembodiment” mean that a particular feature, structure, or characteristicdescribed in connection with the embodiment is included in at least oneimplementation encompassed within the present invention. Thus,appearances of the phrase “one embodiment” or “in an embodiment” are notnecessarily referring to the same embodiment. Furthermore, theparticular features, structures, or characteristics may be instituted inother suitable forms other than the particular embodiment illustratedand all such forms may be encompassed within the claims of the presentapplication.

While the present invention has been described with respect to a limitednumber of embodiments, those skilled in the art will appreciate numerousmodifications and variations therefrom. It is intended that the appendedclaims cover all such modifications and variations as fall within thetrue spirit and scope of this present invention.

What is claimed is:
 1. A photovoltaic system comprising: a firstphotovoltaic module comprising a first module switch, the firstphotovoltaic module operatively coupled with a first circuit, the firstcircuit operatively coupled to an alternating current (AC) mainpanelboard; a second photovoltaic module comprising a second moduleswitch, the at least second photovoltaic module operatively coupled withthe first photovoltaic module through the first circuit; a secondcircuit operatively coupled with the first module switch and with thesecond module switch; a System-Monitor device operatively coupled withthe first circuit, the second circuit, and the AC main panelboard, theSystem-Monitor device comprising a manual switch, the System-Monitordevice configured to generate a System-On signal and to feed theSystem-On signal to the first module switch and to the second moduleswitch through the second circuit when the manual switch is in an onstate; wherein the first module switch is configured to disable thefirst photovoltaic module through one of shorting the first photovoltaicmodule and disconnecting the first photovoltaic module from the firstcircuit when the System-On signal is not received by the first moduleswitch from the second circuit; wherein the second module switch isconfigured to disable the second photovoltaic module through one ofshorting the first photovoltaic module and disconnecting the firstphotovoltaic module from the first circuit when the System-On signal isnot received by the second module switch from the second circuit.
 2. Thesystem of claim 1 wherein the manual switch is including a manuallycontrolled and lockable switch with visual feedback to enable or disablethe System-On signal.
 3. The system of claim 1 wherein theSystem-Monitor device further comprises a sensor switch where theSystem-Monitor device is further configured to feed the System-On signalto the first module switch and to the second module switch through thesecond circuit when both the manual switch and the sensor switch are inthe on state.
 4. The system of claim 3 wherein the sensor switchcomprises a sensor selected from the group consisting of motion,moisture, heat sensors, and any combination thereof.
 5. The system ofclaim 1, wherein the first module switch comprises a current limitingresistor that provides power to an opto-isolated FET driver integratedcircuit, which provides a gate voltage to a FET wherein the FET isconnected to the first circuit.
 6. The system of claim 5 wherein aninherent capacitance of a gate of the FET stores enough charge to remainat a high enough voltage to allow the FET to remain on during an entireAC cycle of the System On signal.
 7. The system of claim 6 including aresistor to protect the FET as the first module switch transitions froman on state to an off state and to drain the charge in time to avoidoverheating due to power dissipation.
 8. The system of claim 1 includinga first isolation device comprised in the first module switch and asecond isolation device comprised in the second module switch.
 9. Thesystem of claim 8 wherein the first isolation device and the secondisolation device are one of a transformer and an optical isolator.
 10. Aphotovoltaic system comprising: a first photovoltaic module comprising afirst module switch, the first photovoltaic module operatively coupledwith a first circuit, the first circuit operatively coupled to analternating current (AC) main panelboard; a second photovoltaic modulecomprising a second module switch, the at least second photovoltaicmodule operatively coupled with the first photovoltaic module throughthe first circuit; a second circuit operatively coupled with the firstmodule switch and with the second module switch; a System-Monitor deviceoperatively coupled with the first circuit, the second circuit, and theAC main panelboard, the System-Monitor device comprising a sensorswitch, the System-Monitor device configured to generate a System-Onsignal and to feed the System-On signal to the first module switch andto the second module switch through the second circuit when the sensorswitch is in an on state; wherein the first module switch is configuredto disable the first photovoltaic module through one of shorting thefirst photovoltaic module and disconnecting the first photovoltaicmodule from the first circuit when the System-On signal is not receivedby the first module switch from the second circuit; wherein the secondmodule switch is configured to disable the second photovoltaic modulethrough one of shorting the first photovoltaic module and disconnectingthe first photovoltaic module from the first circuit when the System-Onsignal is not received by the second module switch from the secondcircuit.
 11. The system of claim 10, wherein the System-Monitor devicefurther comprises a manual switch that is lockable.